Tag Archives: Paleokarst

The St. Peter Sandstone

Recent mapping adventures have reminded me just how much I enjoy studying the St. Peter Sandstone.  This sandstone was named by Arkansas’ first State Geologist, David Dale Owen, for exposures on the St. Peter River, now called the Minnesota River, in southern Minnesota.  The sandstone is Middle Ordovician in age (around 460 million years old) and during that time it extended all the way from Minnesota into Texas.

It is easy to recognize the St. Peter Sandstone whether you are in Minnesota or Arkansas – clean, sugary, white sandstone.  In fact, here is a photo taken from the place where it was first described, known as the type section, under a bridge near Fort Snelling, in St. Paul, Minnesota.

type section st. peter

St. Peter Sandstone in its type area. 

In Minnesota the sandstone easily falls apart.  In Arkansas, the surface of the outcrop is case- hardened meaning there is a hard rind on the rock that forms due to iron-rich water percolating through it and depositing iron on the surface as the water evaporates.  Where this rind is broken, the sand grains fall apart more easily, as at the type area.

st. peter at spring parking (2)

The St. Peter Sandstone cropping out in the parking area for the spring at Blanchard Springs Recreation Area.

The contact of the St. Peter Sandstone with the underlying Everton Formation is particularly interesting.  It is unconformable which means there was a period of non-deposition and erosion before the St. Peter was deposited.  It is also undulatory with as much as 20 feet of relief in Arkansas.  The relief is the difference between the top and bottom of an undulation.  Flint, 1956, reports that these undulations can reach up to 200 feet in Wisconsin.  Pretty amazing!

st pete everton unconformity 3

In the photo above, the relief at the contact is probably around 5-6 feet.  Note the rock hammer for scale.  The rock above the hammer is the St. Peter Sandstone while the rock the hammer is resting on is the Everton Formation.  Also notice the curvature of the contact.  The reason for the unconformable undulating contact is that the sand in the St. Peter was deposited upon the karsted Everton surface.  Karst forms when rock such as limestone is exposed to slightly acidic rainwater or groundwater and develops sinkholes, caves, and enlarged fractures. Since this karst surface has been buried by the St. Peter Sandstone, it is considered paleokarst.

The geologic story goes something like this.  After the sea that deposited the Everton Formation retreated, the limestone at the top of the formation was exposed.  Weathering and erosion lasted for up to tens of millions of years, during which time an extensive karst surface developed (Palmer and Palmer, 2011).   Sand was brought into the area from the source area to the north (Great Lakes region) by rivers and wind. Later, as the sea advanced again, it spread the sand over the area filling in the depressions and forming a thick deposit covering a large portion of the mid-continent.

inked739 contact 2_li

In this photo the relief is approximately 18 feet.  Note the 6-foot-tall geologist for scale.  The red line indicates the contact between the St. Peter above and the Everton below.

The St. Peter Sandstone is relatively resistant to erosion compared to the surrounding rocks; therefore, it is usually a bluff-former.  The tallest bluff I have seen crops out at Blanchard Springs Recreation Area near the group camp and the amphitheater.

Enjoy these photos of the St. Peter Sandstone and hope to see you in the field!

Angela Chandler

st. peter ss amphitheater

Tall (approximately 70 feet tall) St. Peter bluff behind the amphitheater at Blanchard Springs Recreation Area.

590 st. peter bluff-2

    St. Peter Sandstone bluff near Blanchard Springs Recreation Area. 

dipping st. pete swimming area

The St. Peter Sandstone dipping to creek level at the swimming area in Blanchard Springs Campground.     

References and other sources on the St. Peter Sandstone:

Flint, A.E., 1956, Stratigraphic relations of the Shakopee Dolomite and the St. Peter Sandstone   in southwestern Wisconsin: Journal of Geology, vol. 64, no. 4, pp. 396-421.

Giles, A.W., 1930, St. Peter and older Ordovician sandstones of northern Arkansas:  Arkansas Geological Survey Bulletin 4, 187 p.

Palmer, A.N. and Palmer, M.V., 2011, Paleokarst of the USA:  A brief review; in U.S. Geological   Survey Karst Interest Group Proceedings, Fayetteville, Arkansas:  U.S. Geological Survey Scientific Investigations Report 2011-5031, pp. 7-16.

Geo-pic of the week: Fracture-Fill at Shine-Eye

DSCN5369

The photo above shows a vertical dark rock in the center of flat-lying white rock. The dark rock is a sandstone deposit, probably Mississippian-aged, and the white rock is Silurian-aged limestone. If one were to follow the sandstone dike upward, it would lead to a sandstone bed sitting on top of the limestone. Since the limestone was deposited first, we can infer that it was exposed to weathering. The limestone was solutioned and deep fractures or cracks formed. Afterwards, sand was deposited in the area, filled the fractures in the limestone, and eventually lithified into sandstone. There are several of these sandstone-filled fractures present along the Buffalo National River in Silurian-aged limestone. The one pictured above is located at Shine-Eye.

Sandstone Paleokarst

If you have spent any time on Beaver Lake in northwestern Arkansas, then you have probably seen sandstone paleokarst features.  Some stand tall like towers while others appear to be irregular to rounded masses.  It is common to see only the tops of these features when the lake level is low to normal.

 ss paleokarst photo    Top of sandstone mass in Beaver Lake.  Photo taken in October, 2016.

ss paleo 2-01  Sandstone mass along Beaver Lake.  Photo taken in October, 2016.

These features have been in geology literature since 1858 when David Dale Owen made his first geological reconnaissance of the northern counties.  He described a mass of isolated sandstone within adjacent magnesian limestone (now called dolostone) which stands out forming a conspicuous feature in the landscape.  Purdue, 1907, called them cave-sandstone deposits and was the first to consider them paleokarst.  Purdue and Miser, 1916, noted many of these deposits and concluded several were ancient sinkholes that had been filled with sand.  Two theses that pre-date the construction of Beaver Lake (Arrington, 1962, and Staley, 1962) mention numerous sandstone bodies within the Powell.  One very large sandstone mass was seen in the White River (Arrington, 1962).  It is approximately 45 feet tall!  Unfortunately, it is now covered with water.

photo       Sandstone mass in Carroll County.  From Owen, 1858

photo2 Sandstone mass in the White River near Hwy 12 access to Beaver Lake.  From  Arrington, 1962.

So how did these features form?  First, let’s define paleokarst.  Paleokarst consists of karst features that formed in the geologic past and were preserved in the rock record.  Karst features include sinkholes, springs, and caves.  These features form when acidic rain and ground water dissolves carbonate rocks (mainly rocks that contain calcium carbonate – calcite, or calcium-magnesium carbonate – dolomite).

The majority of sandstone masses are surrounded by dolostone, composed of dolomite, in the Powell Formation.  The Powell is Lower Ordovician in age, meaning it formed around 470 million years ago (mya).  It is likely that this formation was exposed to weathering at that time.  Depressions of various size, called sinkholes, developed on the exposed land surface.  Later, sand filled the depressions and eventually became rock called sandstone.  The age of the sandstone masses ranges from Middle Ordovician (approx. 450 mya) to Middle Devonian (approx. 390 mya).  Therefore, there is a gap in the rock sequence, between dolostone in the Powell and the sandstone, called an unconformity, lasting from 20-80 million years.

ss mass 3-01Sandstone mass (center) surrounded by Powell dolostone along Beaver Lake.  Photo taken in September, 2016.

Why is paleokarst important, other than being interesting features to observe?  Paleokarst provides clues to former geologic conditions and changes in climate and sea level (Palmer and Palmer, 2011).  We know that sea level was high in the Lower Ordovician and shallow seas covered all of northern Arkansas.  But, in the Middle Ordovician, sea level lowered and the sandstone paleokarst features provide additional evidence supporting this change.

Many sandstone paleokarst features were located while mapping the War Eagle quadrangle.  Fifty-two sandstone masses were located around Beaver Lake.  This is not a complete list, however, since the main focus of mapping was not a paleokarst inventory.

paleokarst points    Sandstone masses (yellow) located from recent geologic mapping around Beaver Lake.

The War Eagle quadrangle was mapped in preparation for State Park Series 4 – Geology of Hobbs State Park.  Follow the link below to see the geologic map of the War Eagle quadrangle:  http://www.geology.ar.gov/maps_pdf/geologic/24k_maps/War%20Eagle.pdf.

Until next time,

Angela Chandler

 

References:

Arrington, J., 1962, The geology of the Rogers quadrangle:  University of Arkansas M.S. Thesis, 61 p.

Palmer, A.N., and Palmer, M.V., 2011, Paleokarst of the USA:  A brief review:  in U.S. Geological Survey Karst Interest Group Proceedings, Fayetteville, Arkansas:  U.S. Geological Survey Scientific Investigations Report 2011-5031, p. 7-16.

Owen, D.D, 1858, First report of a geological reconnaissance of the northern counties of Arkansas made during the years 1857 and 1858:  Little Rock, 256 p.

Purdue, A.H., 1907, Cave-sandstone deposits of the southern Ozarks:  Geological Society of America Bulletin, vol. 17, pp. 251-256.

Purdue, A.H., and Miser, H.D., 1916, Geologic Atlas of the United States, Eureka Spring-Harrison Folio, Arkansas-Missouri:  U.S. Geological Survey Folio No. 202, 82 p.

Staley, G.G., 1962, The geology of the War Eagle quadrangle, Benton County, Arkansas:   University of Arkansas M.T. Thesis, 56 p.

 

 

 

Geopic of the week: Paleokarst on the Buffalo River

Big Plug Paleosinkhole Gimped 20 Apr 01

This is a picture of a paleokarst feature from the Upper Buffalo River in Newton County, Arkansas.  Paleokarst features, like this one, are ancient caves or sinkholes that have been preserved in the rock record.

In this case, a sinkhole formed when bedrock was exposed above sea level and acidic rainwater dissolved a vertical pit in the bedrock.  When sea level rose and covered the area again, more sediment was washed in and the sinkhole was filled with sand.  Eventually the sand became sandstone and a cast of the sinkhole is preserved today (center of photo).

All of this happened about 450 million years ago.  Paleokarst features are one more clue geologists use to decipher earth’s history.  If you didn’t know better, you might float right by and never give it a second thought.